1. Technical Field
The present invention relates to a liquid crystal device, a driving method thereof, and an electronic apparatus.
2. Related Art
In the past, there have been known liquid crystal devices in which a liquid crystal layer is disposed between pixel electrodes and counter electrodes. Each pixel electrode is electrically connected to a switching element such as a thin film transistor (hereinafter referred to as a TFT). The switching element is controlled to be turned ON/OFF by the input of the scan signal transmitted from the scan line. In the ON state, the switching element applies a voltage, which is transmitted from the data line, to the pixel electrode. An electric field is applied between the pixel electrode and the counter electrode by the voltage, and the liquid crystal layer is driven by the electric field.
A normal liquid crystal device employs, for example, inversion drive (AC drive) which inverts the polarity of the driving voltage applied to each pixel electrode for each scan line, each data line, or each frame in an image signal. That is, the liquid crystal layer is driven by AC. In order to drive the liquid crystal layer through AC, for example, the counter electrode is maintained at a predetermined counter electrode potential, and the electric potential of the pixel electrode is changed between a high potential (positive polarity) and a low potential (negative polarity) relative to the counter electrode potential during the duration of two successive frames. In such a manner, the direction of the electric field applied to the liquid crystal layer is inverted, and thus it is possible to reduce bias of the electric charges of the liquid crystal layer.
When the bias of the electric charges is reduced, it is possible to reduce a DC voltage component which is applied to the liquid crystal layer by the bias of electric charges, and thus it is possible to suppress the occurrence of display defects. Specifically, since the balance between the amount of positive charge and the amount of negative charge is prevented from being broken by the DC voltage component, flicker in the display image, caused by change in transmittance of the liquid crystal device during positive and negative polarity durations, rarely occurs. Further, due to the DC voltage component, it becomes difficult to display normal patterns formed by normally applying an electric field to the liquid crystal layer (image persistence). However, the way of simply performing the inversion drive is not enough to completely solve the problem based on the application of the DC voltage component, and there are still display defects. That is, even when the inversion drive is performed, the DC voltage component is applied to the liquid crystal layer, and the bias of the electric charges occurs, and it is therefore necessary to cope with this.
However, it has been known that, when the liquid crystal device is driven in a state where the electric potential difference between the counter electrode potential and a high potential is made to be equivalent to the electric potential difference between the counter electrode potential and a low potential, the DC voltage component occurs. It can be inferred that the DC voltage component is caused by the following two phenomena.
The first phenomenon is a phenomenon (called a field-through, a pushdown, or a punch-through phenomenon) that causes change in the electric potential of the pixel electrode since electric charges of the channel area are distributed and the pixel electrode is charged therewith when the switching element is switched from the ON state to the OFF state. Specifically, the phenomenon is a phenomenon that lowers the voltage of the pixel electrode since the electric charges, which are accumulated in the parasitic capacitance and the storage capacitance, are redistributed at the time of turning off the switching element.
The second phenomenon is a phenomenon that causes the bias of electric charges since the electric characteristics are asymmetric on the pixel electrode side and the counter electrode side of the liquid crystal layer.
The problems due to the occurrence of the DC voltage component caused by the first phenomenon can be solved by measuring or estimating, in advance, the amount of change in the electric potential of the pixel electrode caused by the parasitic capacitance of the switching element and by setting the counter electrode potential so as to cancel the change in the amount of positive charge and the amount of negative charge caused by the amount of change in electric potential.
An exemplary technique for solving the problems due to the occurrence of the DC voltage component caused by the second phenomenon is disclosed in JP-A-2007-219356.
The liquid crystal device in JP-A-2007-219356 includes a tilted-homeotropic-alignment liquid crystal which is sandwiched between a first inorganic alignment film and a second inorganic alignment film, and a voltage applying member. The thickness of the second inorganic alignment film is more than the thickness of the first inorganic alignment film. The voltage applying member applies a predetermined voltage, which is for setting the first inorganic alignment film side to a first electric potential and setting the second inorganic alignment film side to a second electric potential which is lower than the first electric potential.
In the technique disclosed in JP-A-2007-219356, the electric potential is set to be different between the first inorganic alignment film side and the second inorganic alignment film side. Thereby, it is expected to obtain an effect that relaxes the bias of electric charges due to the thickness difference between the first inorganic alignment film side and the second inorganic alignment film side. However, the bias of electric charges may be caused from factors other than the thicknesses of the first inorganic alignment film and the second inorganic alignment film. Therefore, in terms of effectively reducing the DC voltage component in accordance with the configuration of the liquid crystal device, there is room for improvement in the technique disclosed in JP-A-2007-219356.
Further, there have been proposed methods of driving the liquid crystal device focusing on the above-mentioned two phenomena. For example, JP-A-2002-189460 discloses a technique of shifting the counter electrode potential as a reference of the polarity inversion in advance so as to correct the effect caused by the first phenomenon (the field-through phenomenon) and the second phenomenon (the change in voltage caused by the electrical characteristic difference between the element substrate and the counter substrate). Specifically, in JP-A-2002-189460, the amount of change in voltage caused by the first phenomenon and the amount of change in voltage caused by the second phenomenon at the initial stage is measured on the basis of a predetermined measurement condition, and a value obtained by adding those is added as a regular correction voltage to the set potential (VCOM) of the counter electrode.
In the technique disclosed in JP-A-2002-189460, by applying the correction voltage, to which the amounts of change in voltage caused by the first and second phenomena are added, to the counter electrode potential, it is expected to suppress deterioration in display quality caused by the occurrence of the DC voltage component.
As it is, in a case where the correction voltage of the second phenomenon has a certain magnitude relative to the correction voltage of the first phenomenon, the counter electrode potential may be drastically shifted to the positive or negative side. In other words, when the correction voltage for the second phenomenon is large, an amplitude difference in positive and negative driving voltages increases. Hence, in some cases, display defects such as flicker may occur.